In the present paper, the existence and location of the type series of the Japanese dancing mouse or waltzer, Mus wagneri variety rotans Droogleever Fortuyn, 1912, are established, and a lectotype is designated. Available type specimens are measured, and some morphological parameters, sex, and general condition of the specimens are recorded. A literature survey was conducted, and an attempt is made to clarify the position of M. wagneri variety rotans in the taxonomy of Mus. A genetic analysis suggests that the type series of the Japanese dancing mouse represent a crossbred, or derivation of a crossbred, between the original Japanese dancing mouse of Mus musculus molossinus Temminck 1844 origin and European fancy or laboratory mice of Mus musculus domesticus Schwarz & Schwarz, 1943 origin. Much of their genome was replaced and occupied by Mus musculus domesticus type genome, probably through extensive breeding with European mice.